I always check out the refurb store to see what Apple are selling a similar machine for, then you have to price up or down depending on what warranty you have (of course the Apple refurb machines have the standard 1 year warranty - you may have 2-3 years coverage if you purchased Applecare)

For example, I just checked the refurb site and they have the following system available for £921.20 including shipping and 1 yr Warranty:

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